Fix SET object/block block assuming /any (#1763)

SET without /any wasn't triggering the error it should have when
assigning a block of values that include unsets to a block or an
object. This tests for that condition and triggers the error before
even a single assignment happens, all or nothing.

See http://issue.cc/r3/1763 for the ticket.

Also added documentary comment about new unwind policy. The unwind
screening required to fix http://issue.cc/r3/1515 must be performed
elsewhere, in Set_Var, Do_Next and other related functions.